1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, this invention is directed to preventing lithium from bridging between the positive and negative portions of a cell during discharge, particularly high rate intermittent pulse discharge. Such lithium bridging is referred to as a “lithium cluster” and should it occur, an internal loading mechanism that prematurely discharges the cell could result.
2. Prior Art
The mechanism controlling lithium deposition between the positive and negative cell portions of a case negative primary lithium electrochemical cell, such as between the cathode lead and casing, is described in the publication by Takeuchi, E. S.; Thiebolt, W. C. J. Electrochem. Soc. 138, L44-L45 (1991). While this report specifically discusses measurements made on the lithium/silver vanadium oxide (Li/SVO) system, it also applies to other solid insertion type cathodes used in lithium cells where voltage decreases with discharge.
According to the investigators, lithium deposition is induced by a high rate intermittent discharge of a Li/SVO cell and can form “clusters” bridging between the negative case and the positive connection to the cathode. This conductive bridge can then result in an internal loading mechanism that prematurely discharges the cell.
The mechanism for lithium cluster formation is as follows: at equilibrium, the potential of a lithium anode is governed by the concentration of lithium ions in the electrolyte according to the Nernst equation. If the Li+ ion concentration is increased over a limited portion of the electrode surface, then the electrode/electrolyte interface in this region is polarized anodically with respect to the electrode/electrolyte interface over the remaining portion of the electrode. Lithium ions are reduced in this region of higher concentration and lithium metal is oxidized over the remaining portion of the electrode until the concentration gradient is relaxed. The concentration gradient is also relaxed by diffusion of lithium ions from the region of high concentration to low concentration. However, as long as a concentration gradient exists, deposition of lithium is thermodynamically favored in the region of high lithium ion concentration.
In a Li/SVO cell, Li+ ions are discharged at the anode and subsequently intercalated into the cathode. The anode and cathode are placed in close proximity across a thin separator. Immediately after a pulse discharge, the Li+ ion concentration gradient in the separator is dissipated as the Li+ ions diffuse the short distance from the anode to the cathode and then within the pore structure of the cathode. However, at the electrode assembly edge, the anode edge is not directly opposed by the cathode edge. If excess electrolyte pools at this edge, Li+ ions, which are discharged into the electrolyte pool, have a longer distance to diffuse to the cathode than Li+ ions discharged into the separator. Consequently, this electrolyte pool maintains a higher concentration of Li+ ions for a longer period of time after the pulse discharge.
Typically, the lithium anode tab is welded to the inside of the casing. Therefore, if these components are also wetted by excess electrolyte, this concentration gradient extends over the tab and casing, and lithium cluster deposition is induced onto these surfaces by the Nernstian anodic potential shift derived from the higher Li+ ion concentration in the excess electrolyte pool after the pulse discharge.